Velocity–conductivity relationships for mantle mineral assemblages in Archean cratonic lithosphere based on a review of laboratory data and Hashin–Shtrikman extremal bounds

Abstract

Can mineral physics and mixing theories explain field observations of seismic velocity and electrical conductivity, and is there an advantage to combining seismological and electromagnetic techniques? These two questions are at the heart of this paper. Using phenomologically-derived state equations for individual minerals coupled with multi-phase, Hashin-Shtrikman extremal-bound theory we derive the likely shear and compressional velocities and electrical conductivity at three depths, 100 km, 150 km and 200 km, beneath the central part of the Slave craton and beneath the Kimberley region of the Kaapvaal craton based on known petrologically-observed mineral abundances and magnesium numbers, combined with estimates of temperatures and pressures. We demonstrate that there are measurable differences between the physical properties of the two lithospheres for the upper depths, primarily due to the different ambient temperature, but that differences in velocity are negligibly small at 200 km. We also show that there is an advantage to combining seismic and electromagnetic data, given that conductivity is exponentially dependent on temperature whereas the shear and bulk moduli have only a linear dependence in cratonic lithospheric rocks.Focussing on a known discontinuity between harzburgite-dominated and lherzolitic mantle in the Slave craton at a depth of about 160 km, we demonstrate that the amplitude of compressional (P) wave to shear (S) wave conversions would be very weak, and so explanations for the seismological (receiver function) observations must either appeal to effects we have not considered (perhaps anisotropy), or imply that the laboratory data require further refinement. Jones et al.Velocity-conductivity relations of Archean lithospheric mantle

Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

“…Various definitions exist for the continental LAB (chemical, thermal, mechanical, seismic, electrical;Artemieva and Mooney, 2001;Eaton et al, 2009), implying that the thickness of cratonic roots can be measured by a variety of geochemical, petrologic and geophysical means. Compositionally, the LAB is typified by a transition from depleted to fertile (or refertilised) peridotite that has been mapped using xenoliths and xenocrysts (Gaul et al, 2000;Kopylova and Russell, 2000;Griffin et al, 2002Griffin et al, , 2004.…”

Imaging the electrical lithosphere of South Australia - 2D profiles and preliminary results of AusLAMP SA

“…Geochemically, the transition results in a thinning of refractory, iron poor sub-continental lithosphere (Griffin et al, 2009). Electrical laboratory measurements of main mantle constituting minerals suggest an increase in conductivity due to rehydration and fertilisation processes (Jones et al, 2009a). This sub-grid is placed such that it will be able to constrain the laboratory measurements in the field in conjunction with AusLAMP MT data across Victoria.…”

The CAFE Experiment : a joint seismic and MT investigation of the Cascadia Subduction System

“…While much of the geophysical exploration of the region has involved seismology, the magnetotelluric method has also been employed to good effect. Magnetotellurics is attractive because while it is sensitive to a completely different set of rock properties than are seismic methods (Hellfrich, 2003, Jones et al, 2009), velocity and conductivity are often correlated (Stanley et al, 1990, Jones et al, 2009), either directly or simply in the fact that at distinct geological boundaries both parameters are likely to change (Gallardo, 2007, Moorkamp et al, 2007. It is therefore tempting to utilize both types of data in our interpretations, particularly where the data profiles are collocated (Gallardo and Meju, 2007).…”

“…The rationale behind joint inversion of multiple data sets is to improve resolution, diminish the influence of noise, and reduce the inherent non-uniqueness by providing additional constraints (Gallardo, 2007, Moorkamp et al, 2007. While the values of velocity and resistivity have been compiled for a wide range of geological and physical conditions , Jones et al, 2009, there exists no fundamental relationship between the two physical properties (Bedrosian et al, 2007).…”

“…Secondly, because velocity and conductivity are often in fact correlated (Stanley et al, 1990, Jones et al, 2009) either directly or simply in the fact that at distinct geological boundaries both parameters are likely to change (Gallardo, 2007, Moorkamp et al, 2007, Jones et al, 2009), the two studies will provide some independent confirmation of one another. Finally, the two techniques can be used in conjunction to emphasize the strengths and mitigate the weaknesses of the other.…”